1. Field of the Invention
The present invention relates to an improved expansion shell assembly for mine roof bolts and, more specifically, to an improved expansion shell assembly, and elements thereof, having novel features particularly adapted for the improved support and release of an expansion shell during installation of a mine roof bolt.
2. Description of the Art
It is well known in the art of mine roof control to tension bolts anchored in bore holes drilled into the mine roof in order to reinforce the unsupported rock formation above the roof. Conventionally, a hole is drilled into the rock formation. The end of the bolt in the rock formation is anchored either by engagement of an expansion shell assembly on the end of the bolt with the rock formation, by bonding the bolt with resin to the rock formation surrounding the bore hole, or by use of both an expansion shell assembly and resin together to retain the bolt within the hole.
Mechanical expansion shell assemblies for roof bolts have been used for many years in the anchorage of bolts in rock formations. An expansion shell assembly includes an expansion member such as a camming plug or tapered plug threaded onto one end of a mine roof bolt and positioned within the upper end portion of an expansion shell. The expansion shell is held in place by a support device such as a PALNUT positioned adjacent to the lower end portion of the expansion shell.
During rotation of the bolt, the frictional engagement of the expansion shell with the rock formation surrounding the bore hole prevents rotation thereof. Also, by virtue of the frictional engagement between the upper end of the support device with the lower end of the expansion shell, the support device will not rotate but, upon rotation of the bolt, the support device, the expansion shell and the tapered plug move downwardly along the bolt until the support device reaches the unthreaded portion of the bolt.
Continued rotation of the bolt causes the tapered plug to advance downwardly on the bolt and urges the expansion shell fingers to expand or deflect radially outwardly to grip the rock formation surrounding the bore hole. With the expansion shell engaged with the rock formation, continued rotation of the bolt causes the bolt to advance upwardly, thereby pushing or stripping the support device off the threaded portion of the bolt. Concurrently, rotation of the bolt urges a bearing plate positioned on the bolt at an end opposite the expansion shell assembly against the rock formation, putting the rock formation in compression and the bolt in tension.
Current practice provides roof bolt systems using resin in conjunction with a mechanical anchor. These types of systems require a resin cartridge to be inserted in the bore hole. The bolt is then inserted in the bore hole and thrust upwardly to rupture the resin cartridge. The bolt is rotated, mixing the resin and setting the mechanical anchor. Two of the advantages of resin-using systems are a tensioned resin anchor and quick installation provided by the mechanical anchor.
Examples of arrangements utilizing both an expansion shell assembly and resin to anchor a mine roof bolt in a rock formation are disclosed in U.S. Pat. Nos. 4,419,805; 4,413,930; 4,516,885 and 4,518,292, all expressly incorporated herein by reference. Other examples of both an expansion shell assembly and resin to anchor a mine roof bolt are disclosed in U.S. Pat. Nos. 3,188,815; 4,162,133; 4,655,645 and 4,664,561, all expressly incorporated herein by reference.
In general, any support device of an expansion shell assembly of the prior art performs three functions during installation of the roof bolt. First, the support device maintains the expansion shell in engagement with the camming plug during insertion in the bore hole. Second, with the support device positioned adjacent the lower end of the bolt threaded portion, the support device maintains the expansion shell in a fixed axial position on the bolt thereby allowing the tapered plug to advance downwardly on the bolt and urge the expansion fingers radially outwardly to grip the rock formation. Finally, with the expansion shell engaged with the rock formation, the support device strips off the threaded portion of the bolt thereby allowing the bolt to advance upwardly and be properly tensioned.
Some early combination resin/mechanical anchor systems used a PALNUT as the support device to maintain the expansion shell in engagement with the camming plug during insertion in the bore hole. However, because of the tight clearance between the expansion shell fingers and the bore hole wall, along with the high resistance to the flow of resin caused by the anchor, the PALNUT was prone to prematurely strip off the threaded portion of the bolt. When a premature failure occurred, the expansion shell was pushed downwardly and disengaged from the camming plug. The result was an expansion shell assembly unable to engage the rock formation, a condition known in the industry as a “spinner”.
Manufacturers now use a hex or round jamnut of a various thickness as the support device. Although the heavier jamnut solved the premature stripping problem, it created a different problem. It has been determined that the heavier jamnut may not strip off the threaded portion. As a result, rotation of the bolt causes the expansion shell to be compressed between the camming plug and the support device. The expansion shell may become severely distorted, ripped, torn or twisted. In addition, when the jamnut fails to release, significant tension occurs in the bolt threaded portion between the camming plug and the jamnut. This tension translates into torque being measured by the roof bolt installation machine. It is common practice to install a roof bolt to a predetermined desired torque. Since significant torque is being created at the expansion shell assembly, significantly reduced torque/tension is being applied to the roof bolt. Thus, the actual tension in the bolt may be significantly less than the desired tension in the bolt.
While expansion shell assemblies for anchoring mine roof bolts in bore holes are well known, there is need to improve the operability of the support device, particularly when used with resin. The support device must have the capability to support the expansion shell during insertion in the bore hole and engagement with the bore hole wall. At the same time, the support device must have the capability to release its axially supporting engagement with the expansion shell once the expansion shell is set in the rock formation to allow for proper tensioning of the bolt while reducing or eliminating torsion and/or compression between the camming plug and the support device.